The present invention relates generally to satellite communications systems and, more particularly, to a satellite system employing spread spectrum waveform protocols.
Wireless communication is widespread between mobile users for transmitting information such as voice, data, and video. Initially, ground based cells or repeater stations were employed to transmit the information to the mobile users but they limited communication to certain distances between the mobile users and the ground stations. Increasingly, satellites are used to remove such limitations. The use of satellite for wireless communication is common for applications such as mobile phones and global positioning systems.
One problem facing satellite communications systems is the need to accommodate an increasing number of users. The need to achieve network capacity may come at the expense of quality of service. For example, one current method of increasing capacity relies on time division multiple access (TDMA) protocols, which uses a plurality of channels to transmit information. To increase capacity or the number of channels, this method divides a given time frame into more and more slots. Each slot is allowed less time with each successive division. Because the channel data rate has an upper bound determined by the physical characteristics of the satellite and the terminals, with each division a user can send less data. As the number of slots increases, the data rate for each user is decreased. In current military applications this has meant that voice service is being conducted at 2.4 kbps. The commercial standard in the cell phone industry is in the range of 11 kbps to 22 kbps for voice service. Existing wireline voice service is 64 kbps.
The UHF frequency band has been selected for military use. Current mobile terminal units for use with global UHF satellites are heavy, weighing about fifteen pounds or more. The mobile terminal units need large batteries and hardware due to the great power requirement for the signals to reach the current satellites. Because terrestrial users have primary rights to the UHF spectrum and UHF satellite users have only secondary rights, signals between the UHF satellites and ground terminals and units need to be sufficiently low in strength to be considered mere insignificant noise as compared to those for the primary users. That is, the UHF satellite communication needs to meet the earth power flux density limit. The constraints on power and satellite gain render it difficult to reduce the size and weight of mobile terminal units to mobile units comparable to a commercial mobile unit.